The present invention generally relates to thermo-photovoltaic power generators and thermal light emitters and, more particularly, to such generators and light emitters including a thermal source of radiation and a film with openings formed therein for selectively passing predetermined wavelengths of radiation, and reflecting other wavelengths of the radiation.
A conventional incandescent light bulb is about 10% efficient in converting input energy into visible light in the wavelength range of 400-to-750 nm, where most of the input energy is radiated as infrared light with wavelengths longer than 750 nm. FIG. 1 shows the emission spectrum of a blackbody at ˜3000° K simulating that of a tungsten filament in a conventional light bulb. Human eyes are sensitive to light with wavelengths between ˜400 and 750 nm, and a large portion of the emitted light from the tungsten filament is at longer wavelengths than human eyes can detect. About 90% of the input electrical power is converted into these invisible infrared photons, many of which are absorbed in the bulb envelope and thereby heat the envelope. If these longer wavelengths can be reflected back towards the hot filament before reaching the bulb envelope, while allowing the visible wavelengths to pass through the envelope, the unseen heat energy will be re-absorbed by the filament, and less input electric power will be required to maintain visible light output, thus improving the efficiency of the bulb. In the ideal case where infrared reflection is perfect and there is no thermal conduction of heat from the filament to the bulb envelope, the infrared reflecting bulb will be an order-of-magnitude more efficient than a conventional light bulb.
A conventional approach to fabricating a selective long-wavelength reflector, or “hot mirror,” is to use one or more dielectric stacks composed of three layers with alternating indices of refraction. This type of hot mirror is also called a dielectric interference mirror or dichroic mirror. At least three depositions of materials, each with a well-defined thickness requirement to create the desired optical interference, may be needed to produce a conventional hot mirror. A typical single stack dichroic mirror may produce high transmission in the visible wavelength range, but the long wavelength reflection range is not wide enough to reflect most of the spectrum emitted by a 3000° K blackbody. FIG. 2 shows the spectral reflectance of a conventional single stack dichroic mirror. As depicted, the second passband may start at about 1100 nm with additional passbands occurring at even longer wavelengths, failing to reflect most of the IR radiation that extends up to ˜4 microns. Single stack dichroic hot mirrors typically reflect the wavelength range from ˜750-to-1250 nm while advanced multi-stack hot mirrors may reflect from ˜750-to-2000 nm. For a 3000° K black body, single and multi-stack hot mirrors usually reflect about 32% and 62%, respectively, of the total photon energy emitted by a filament.
As the thickness of each layer of the dichroic mirrors determines the wavelength band of the reflected light, each layer needs to be deposited with high precision. Also, the dichroic mirror requires a number of layers to reflect most of the IR energy emitted by a filament. Moreover, each of the multiple layers needs to be uniformly coated on the light bulb surface, which may translate into high manufacturing cost. Thus, there is a strong need for a reflector that can operate as a low-pass filter and can be applied to conventional light bulb design in a cost-effective manner.